1. Field of the Invention
The present invention relates to a pneumatic rotary drive.
2. Description of the Related Art
U.S. Pat. No. 4,838,148 discloses a pneumatic rotary drive in which a shaft and a guide body are arranged within a housing. The guide body is completely surrounded by a ring-shaped closed belt which has an external toothing and engages the shaft which is provided with a complimentary toothing. The ring-shaped belt is placed around the guide body in such a way that the belt defines a kidney-shaped contour. This kidney-shaped contour forms pressure medium chambers. Depending on which side the pressure medium is applied, the ring-shaped belt travels to the one side or the other side until a belt portion rests against one or the other side of the guide body and cannot yield any further. The movement carried out between these end stop positions can be transmitted through the toothings of belt and shaft to the shaft itself, so that a rotary movement of the shaft is produced which can be picked off on the outside. In the ideal case, when the appropriate pressure is applied, the belt should not slide, but rather the kidney shape should unwind in a rolling manner.
However, in the rotary drive known from the above-mentioned U.S. Patent, it may occur that the belt slides through in the region of the portion which rests against the inner wall when pressure is applied and, thus, effectively a smaller unrolling distance of the kidney-shaped belt is available. Consequently, because of the resulting smaller unrolling distance of the kidney contour, effectively also a smaller angle of rotation is available for picking off on the outside. Moreover, there is the disadvantage that, because of the possibility of yielding of the belt, the force which can be applied to the rotary shaft is limited by static friction or, in the most unfavorable case, by sliding friction which is even smaller.
U.S. patent application Ser. No. 07/861,228 proposes a rotary drive of the above-described type which is provided with two guide bodies which are spaced apart from each other and rest with the belt against the outer circumference of the shaft. In this rotary drive, as is the case in the rotary drive known from the above-mentioned U.S. patent, the belt itself is a ring-shaped closed belt. Such a ring-shaped closed belt has the disadvantage that it is relatively complicated to manufacture because it must be manufactured either as a whole by injection molding or, if the belt is to be made from a strip of material, the ends of the belt have to be glued together subsequently in a complicated manner. As a result, the belt and, thus, the rotary drive is complicated and expensive to manufacture.